1. Field of the Invention
The present invention relates to a method of quickly decomposing and removing an organic chlorine compound hazardous to the body, contained in e.g. ground water, river water, water in water supply and sewerage system, industrial waste water or contaminated soil, on a hollandite-type photocatalyst with a small amount of light energy so that the organic chlorine compound undergoes oxidative destruction more quickly and completely as compared with a method of bringing said photocatalyst into contact with an organic chlorine compound in a gaseous state, followed by irradiation with light for decomposition, or a method of bringing said photocatalyst into contact with an organic chlorine compound contained in a liquid by means of e.g. mixing, followed by irradiation with light for decomposition.
2. Discussion of Background
Attention has been paid to organic chlorine compounds as problematic endocrine disturbing chemicals, and risks of their carcinogenicity have conventionally been pointed out. Their representative examples include trichloroethylene and perchloroethylene.
Conventionally, as a method of removing an organic chlorine compound such as trichloroethylene in water, a method of completely oxidizing the organic chlorine compound by means of e.g. a platinum catalyst at a high temperature or a method of completely oxidizing it by means of a titania (TiO2) catalyst at room temperature under irradiation with ultraviolet light, after aspirating and evaporating water by e.g. stripping, has been considered.
It is possible to completely decompose an organic chlorine compound such as trichloroethylene by either of the above two methods, but the treatment is carried out after water is evaporated in these methods, whereby utilizing efficiency of water tends to decrease.
However, in a case where a TiO2 type photocatalyst is used, a method of mixing said photocatalyst with a solution containing an organic chlorine compound, followed by irradiation with light to decompose the organic chlorine compound, or a method of forming the TiO2 type photocatalyst into a thin film, followed by irradiation with light while passing a solution containing an organic chlorine compound on the film to decompose the organic chlorine compound, are proposed. However, in either case, although utilization efficiency of water will improve, intensity of light tends to decay in water, and accordingly the decomposition rate tends to significantly decrease as compared with a method of bringing an organic chlorine compound in a gaseous state into contact with a photocatalyst and decomposing the organic chlorine compound under irradiation with light. Further, no adequate effect of a clarification method by a conventional photocatalyst can be obtained for clarification of contaminated soil.
Under these circumstances, the present invention has been made to overcome the above problems, and provides a method of quickly decomposing and removing an organic chlorine compound hazardous to the body, contained in e.g. ground water, river water, water in water supply and sewerage system, industrial water or contaminated soil, by using a hollandite-type photocatalyst with a small amount of light energy so that the organic chlorine compound undergoes oxidative destruction more quickly and completely as compared with a method of bringing an organic chlorine compound in a gaseous state into contact with said photocatalyst, followed by irradiation with light for decomposition, or a method of bringing said photocatalyst into contact with an organic chlorine compound contained in a liquid by means of e.g. mixing, followed by irradiation with light for decomposition.
The present inventors have conducted extensive studies to develop a photocatalyst having a photocatalytic activity to completely remove an organic chlorine compound contained in water with a small amount of light energy in air, and having excellent durability, and as a result, have found that by a method of coating a liquid containing an organic chlorine compound on the surface of a photocatalyst comprising a hollandite-type crystal phase represented by the formula:
AxMyN8-yO16 
(wherein A is at least one element selected from the group consisting of K, Rb, Cs, Ca, Ba and Na, M is a bivalent or trivalent metal element, N is an element which forms a rutile-type oxide, such as Ti, Sn or Mn, provided that A may be a Na element, only when M is Cr, and x and y satisfy 0.7 less than xxe2x89xa62.0 and 0.7 less than yxe2x89xa62.0, respectively), followed by irradiation with light in air to decompose the organic chlorine compound, an organic chlorine compound undergoes oxidative destruction more quickly and completely as compared with a method of bringing an organic compound in a gaseous state into contact with said photocatalyst, followed by irradiation with light for decomposition, or a method of bringing said photocatalyst into contact with an organic chlorine compound contained in a liquid by means of e.g. mixing or stirring, followed by irradiation with light for decomposition, and that the above method is excellent also in durability.